Edmond Hamilton 
Return to the Stars 

John Gordon, twentieth century Earthman, is torn from his own time to a far distant future – a time when the entire galaxy is inhabited. But men do not rule the future; our race is only one among thousands, and many of those thousands are sworn enemies of humanity! Gordon, man of the past, is forced to form alliances with the men of the future in a desperate battle to save the human race from final annihilation…